The principles of controlling guided missiles are well known. Basic principles are comprehensibly set forth in Locke, Guidance (1955). Since that book was published a wealth of information has been developed to refine and improve upon early missile control techniques, and to accommodate new and ever changing environments. Proportional navigation is discussed at various levels of detail in U.S. Pat. Nos. 3,189,300, 3,223,357, 3,603,531 and 3,718,293.
Earlier techniques worked well for large targets and for targets which were either geographically fixed or were moving relatively slowly and predictably. Such targets were at relatively low altitudes, usually not higher than 80,000 feet, flying at speeds not in excess of Mach 2. For target aircraft of these types, well established means are available in missile guidance computers to provide signals to the missile autopilot which, during terminal guidance, rely upon directly observed boresight error, computed line of sight rate and several other computed and estimated factors, to achieve intercept. Examples of such navigation computers and methods of estimating values used in solving navigational problems are shown in U.S. Pat. Nos. 4,128,837, 4,148,029 and 4,179,696.
However, with high speed, high altitude, maneuvering, small airborne targets, the problems to be solved in order to achieve intercept were greatly increased. In some previously available systems employing noise adaptive gains, the gains were also dependent upon recursive calculations from on-board and possible uplink information. Because of their sensitivity, these systems, often referred to as Kalman systems, tended to degrade rapidly due to unmodeled errors such as randome aberration, especially for high altitude targets. Thus while these systems had potentially optimal accuracy at intercept, errors which could not be modeled into them tended to degrade the resulting accuracy beyond that of conventional proportional navigation systems.
A third order system, sometimes referred to as Hanson's tracker, has been devised with optimal Kalman structure of combined prediction and correction loops for processing the line of sight data for a terminal guidance computer configuration. This was intended to improve over the range aided filtering technique (RAFT), and employed fixed gains. However, for extended range missiles, at high altitudes (generally in excess of 80,000 feet) it has been found that such a configuration has drawbacks which lead to instability or unacceptable miss distances.
Optimal control laws have been known for some time. Theoretically they should provide improved missile terminal guidance. However, to employ optimal control laws to advantage, it is necessary to have optimal estimation of parameters that would affect such control. In actual practice, the results have been disappointing. As a matter of fact, where estimations are less than optimal, the results of using optimal control laws under actual, as opposed to theoretical, conditions has been a degradation of performance in the terminal phase and either instability or increased miss distance, or both. Even a Kalman estimator, with unmodeled errors, is not optimum due to large unmodeled errors and does not support the use of optimal control laws to advantage.